FIELD OF THE INVENTION
The invention relates to a sheet-fed printing machine having an impression cylinder and a movable cylinder that can be thrown onto the impression cylinder and can be moved into a spaced position far removed from the impression cylinder.
A sheet-fed printing machine of this type is described in the published European Patent Document EP 0 477 283 B1. This sheet-fed printing machine has a withdrawable coating device with a coating cylinder which, in relation to an impression cylinder, can be moved into an operating position in order to coat the printing-material sheet resting on the impression cylinder, into a disengaged position at a slight distance from the impression cylinder, and into a position far removed from the impression cylinder and designated a second position.
A disadvantage of this sheet-fed printing machine is that when printing stiff printing-material sheets, for example, sheets made of heavy cardboard, without an in-line coating with the coating device, smearing of the yet-fresh printing ink is not ruled out. If in this case the coating cylinder were to remain in the disengaged position, the printing ink would be smeared onto the coating cylinder, and the printed image would be smeared due to this contact. If the inactive coating cylinder were to remain in the so-called second position, it would then be no longer possible to carry the trailing end of the sheet close to the impression cylinder, because of the inherent stiffness of the printing-material sheet, inasmuch as the coating cylinder located in the second position is no longer able to hold the printing-material sheet on the impression cylinder. The unstable position of the transported printing-material sheet would result in smearing of the printing ink onto other machine parts and possibly even to damage to the printing-material sheet.
The published German Patent Document DE 297 10 252 U1 contains a description of a guide device that is assigned to transfer drums and is provided with guide plates which can be pivoted and moved by pushing or sliding. Although this guide device can be used to carry stiff materials on the sheet transfer drums, and the transfer drums are easily accessible for maintenance and cleaning, the guide device is not suitable for use in conjunction with an impression cylinder and a cylinder that can be thrown onto the latter.
Furthermore, a sheet-fed printing machine that is not of the foregoing general type is described in the published German Patent Document DE 43 18 777 C2. This sheet-fed printing machine has a printing unit with a blanket cylinder and an impression cylinder. When the printing unit is not involved in printing, there is a spacing between the blanket cylinder and the impression cylinder that is about 2 mm distance and is therefore very small. The blanket cylinder of this inactive printing unit, that is located close to the impression cylinder of the unit, cannot be maintained or cleaned during printing with other printing units. One reason for this disadvantage is the poor accessibility of the blanket cylinder in the described operating situation of the printing machine. In addition, the use of two blast tubes or blower pipes arranged in the wedge-shaped space upline and downline of the printing zone is disadvantageous. Although the blast tubes permit a quiet sheet run in the case of thin sheets of paper, this cannot be achieved in the case of thick sheets of cardboard or pasteboard.